To increase the areal storage density of a magnetic recording device, the recording layer thereof may be provided with smaller and smaller individual magnetic grains. This reduction in grain size soon reaches a “superparamagnetic limit,” at which point the magnetic grains become thermally unstable and incapable of maintaining their magnetization. The thermal stability of the magnetic grains can be increased by increasing the magnetic anisotropy thereof (e.g., by utilizing materials with higher anisotropic constants). Increasing the magnetic anisotropy of the magnetic grains, however, increases their coercivity and therefore requires a stronger magnetic field to change the magnetic orientation of the grains (e.g., in a write operation).
Energy-assisted magnetic recording (EAMR) is used to address this challenge. In an EAMR system, a small spot less than ¼λ where data is to be written is locally heated to reduce the coercivity of the magnetic grains therein for the duration of the write operation, thereby allowing materials with increased magnetic anisotropy to be used, and greater areal storage density to be exploited.
In EAMR approach, a semiconductor laser diode is normally used as a light source and coupled to a planar waveguide which serves as light delivery path. A grating structure may be used to couple the laser light into the waveguide. The coupled light is then routed to a near field transducer (NFT) by which the optical energy is provided to a small optical spot on the recording media a few tens of nanometers (nm) in size.
FIG. 1 is a diagram depicting a perspective view of a so-called “Puccini-type” NFT″ 100 comprising a narrow pin section 132 connected to a small disk section 134. FIG. 2 is a diagram depicting a cross-sectional view of an NFT arrangement 200 in which the NFT 100 is coupled to a waveguide structure 210 via an NFT writer gap layer 220. The pin section 132 has pin length 133, and the disk section 134 has disk size (e.g., diameter) 135, and the NFT 100 has NFT thickness 131. The NFT writer gap layer 220 provides writer gap 221 between the waveguide structure 210 and the NFT 100. In the illustrated example, the waveguide structure 220 is a waveguide core layer. Traditional approach for fabricating an NFT arrangement such as 200 of FIG. 2 involves using a milling process to form the pin section 132 followed by a lift-off process to form the disk section 134. For example, the pin section 132 is formed first by ion milling. Second, photolithography is used to form a hole which is aligned with the already-formed pin section 132. Third, the hole is filled with a metal (e.g., Au). Finally, the pin section 134 is formed from the filled metal following a lift-off process.
The traditional fabrication approach and a final NFT structure fabricated thereby suffers from a number of limitations. The writer gap 221 cannot be controlled accurately because during pin milling process, partially-exposed material of the writer gap layer 220 is milled away. In addition, during second NFT photolithography, partially-exposed material of the writer gap layer 220 is also etched away by developer. The writer gap 221 variation depending on over-milling time and photo-rework frequency. The NFT thickness 131 cannot be controlled accurately due to shadow effect and lift off milling process. The NFT thickness 131 variation depends on disk size and photo thickness. The disk size 135 has a lower limit because, with current techniques, a hole formed by the photolithography is limited to a diameter larger than 250 nm. The NFT shape is not consistent since fencing- and bow-shaped surface is typical result of a lift off process.
Accordingly, there is a need for NFT fabrication methods that address the aforementioned limitations associated with the traditional NFT fabrication approach.